To join two metal members, various ultrasonic joining methods are known. For example, Japanese Unexamined Patent Publication No. 2001-246479 discloses an ultrasonic joining method for joining a straight pipe having a flange portion at its first end to a part of a header tank of a heat exchanger.
In this method, the flange portion is vibrated while being pressed against the part of the header tank with a predetermined pressure by a horn in a condition that the heat exchanger is held by an anvil. Thus, oxidation films of opposed surfaces of the flange portion and the part of the header tank are broken due to friction between them. At this time, frictional heat occurs between the contact surface of the flange portion and the part of the header. Also, because newly-formed clean surfaces of the flange portion and the part of the header tank come very close to each other, an atomic force occurs between the surfaces. As such, the flange portion of the pipe is joined to the part of the header tank by the frictional heat and the atomic force.
In this case, the horn has a cylindrical shape so as to surround a periphery of the pipe. Thus, the horn is set on a surface of the flange portion on a side opposite to the header tank through a second end of the pipe, the second end being opposite to the flange portion.
However, if the pipe has a bent or an overhang portion at the second end, it is difficult to set the horn through the second end of the pipe. Therefore, it is proposed to use a horn having a semicylindrical shape. Although this horn can be set to the flange portion from a radial outside of the pipe, it is necessary to perform the ultrasonic joining part by part in a circumferential direction of the flange portion. In this case, when a first part of the flange portion is pressed for joining, a second part that is not joined yet will be deformed. This affects joining of the first part. Also, the first part, which has been already joined, will be damaged by vibration while the second part is joined.
Also, it is also proposed to divide the cylindrical horn into plural horn segments in the circumferential direction in Japanese Patent Application No. 2005-99329. The horn segments are arranged with clearances between them in the circumferential direction, and respectively connected to oscillators. The horn segments are vibrated by the respective oscillators at the same phase and amplitude.